To mount electronic components, particularly large-area semiconductor bodies, the prior art involves the application of different assembly methods. The material-joining assembly techniques on which these assembly methods are based include welding, soldering, sintering and bonding, for example. Through the further development of conventional methods and the development of new methods, assembly engineering has been able to be matched to the increasing demands of modern products, particularly including microelectronics, and the spectrum of application has been able to be continually expanded. The particular significance of assembly engineering for the product properties which are to be attained is based on the need for the joints to meet demands which correspond to the functional properties of the assembly parts. In particular, this also requires assembly-compliant design and optimized precision and also durability of the tools used in the assembly methods.
For producing joints, the prior art typically involves the application of pressure- and temperature-controlled connection techniques, such as die bonding, low-temperature connection technique or diffusion soldering (see EP 1030349, EP 0790647, U.S. Pat. No. 6,727,587, DE 19531158, EP 275433, EP 242626, EP 330895, U.S. Pat. No. 5,379,942, for example).
In this case, a permanent and heat-resistant metal connecting layer is formed between the electronic component and a substrate by means of assembly with the application of a high pressure and a temperature which is dependent on the method applied. To this end, a solder or sinter material is applied prior to the assembly operation either on the substrate or on the side of the electronic component. Examples of possible methods for applying such a layer are dispensing, sputtering, vapor deposition, screen printing or spraying processes. The components prepared in this manner are then placed on top of one another with an exact fit in the assembly operation and are connected to one another by means of a statically applied pressure and an assembly temperature. In this case, the solder or sinter layer is converted into a usually high-melting-point connecting layer between the electronic component and the substrate. Through the formation of intermetal phases, this layer produces a nondeformable connection between the substrate and the electronic component.
During diffusion soldering, metal atoms of the solder diffuse into the workpiece and conversely from the workpiece into the solder. This produces an alloy zone. A prerequisite for diffusion soldering is that the metals used are miscible. The diffusion is heavily temperature-dependent, that is to say the higher the soldering temperature the higher the degree of diffusion and the broader the alloy zone, with a high degree of diffusion having a correspondingly positive effect on the strength values of the connection, such as shear strength.
Besides the electronic functions, a joint between an electronic component and the substrate also needs to have the property of being a good dissipater of the heat produced on an electronic component and of absorbing thermomechanical stresses, for example. In the case of electronic components such as power semiconductors, the demands relating to these properties are particularly high. Thus, a modern IGBT (Insulated Gate Bipolar Transistor) turns off at 1.7 kV, for example, and can have a charging current of 75 A at an on-state voltage of 3.8 V. When the switching losses are additionally taken into account, it must be possible in typical use to dissipate a power loss of around 500 W in the form of heat.
The electronic component itself, which in this case is a source of heat, has a size of only about 1.3 cm×1.3 cm, for example. The electronic component and the materials surrounding this component need to be able to withstand a constant change of temperature from low ambient temperatures down to −55° Celsius, for example, and operating temperatures up to approximately +150° Celsius. These high demands cannot be met by all connection techniques which are customary on the basis of the prior art, which is why in these cases the method known as low-temperature connection technique is used. This method is based on the pressure-sintering of particular silver powders at a low temperature, in comparison with other methods, of approximately 250° Celsius and a moderate pressure of approximately 40 MPa for an action time of approximately 3 minutes. The areas to be connected need to be metallized with gold or silver, for which the aforementioned dispensing, sputtering, vapor deposition, screen printing or spraying methods are used.
In the course of this method, a very stable, sponge-like connecting layer with a high level of resistance to heat and electrical conductivity is produced between the electronic component and the substrate. The advantages of this connection technique over other methods such as soldering and bonding are, inter alia, the absence of a liquid phase during the operation itself, since this involves a solid-state reaction, high strength even above the process temperature (around 250° Celsius), a bubble-free connecting layer, greatly reduced thermomechanical stresses, high load change strength and high electrical and thermal conductivity.
Since all the methods applied on the basis of the prior art involve the thickness of the connecting layer which is to be formed between the electronic components and a substrate being in the micron range, plane-parallel orientation of the surfaces which are to be connected relative to one another is crucial to the success and quality of the connection attained. In addition, many of the electronic components used today on the basis of the prior art have a surface topography which makes the even introduction of force into the surface of these components more difficult. For this reason, the prior art involves the use of plastics in conjunction with assembly tools, so as not to damage the components during the assembly operation.
In this case, these plastics are intended, by virtue of their elasticity, to be able to compensate for tilts and irregular surface topographies of electronic components to a limited degree. One example of this is the use of a silicone cushion for the aforementioned low-temperature connection technique. Such a method is described in the laid-open specification EP 330895, for example. This describes the pressure sintering of components with a patterned surface topography, the electronic components being placed, together with a body made of elastically deformable material, for example silicone rubber, into a receiving chamber which is closed off by a movable die and which transmits the sinter pressure. In this context, upon the sinter pressure being reached, the deformable body fills the remaining interior of the receiving chamber completely and, despite an irregular surface topography of the components which are to be connected to the substrate, is intended to apply the pressure required for the assembly operation nondestructively.
The advantages of using elastic assembly tools, such as the possibility of compensating for a patterned surface topography, are restricted by the fact that there is a lack of long-term stability in these plastic materials, however. One disadvantageous effect in this context is that particularly the influence of the assembly temperatures, which may be up to 400° Celsius depending on the method used, and the influence of mechanical abrasion in the course of the lifetime of such an assembly tool result in a decrease in elasticity and permanent deformation of the surface.
This means increased abrasion of the assembly tool and hence continuous alteration of the assembly parameters in the course of making large numbers of connections between components and the substrate, which means that it is not possible to observe constant and reproducible production and quality parameters during the production process. In addition, with flexible materials such as silicone cushions, tilts in the surfaces between the component which is to be connected to the substrate and the substrate can be compensated for only inadequately.